1. Field of the Invention
This invention relates to labyrinth stator seals particularly those having an abradeble layer thereon.
2. The Prior Art
Labyrinth (lab) seals are used to minimize gas leakage between rotating and static parts in gas turbine engines. In order to minimize steady state clearances, the initial clearance between knife edge and honeycomb is set very tightly, which usually results in a rub therebetween during transient engine operation. Thus in the typical lab seal knife edge rotor - honeycomb stator, the knife edge sets its own running clearance by wearing grooves into the honeycomb. Because of such rubbing, the rotor knife edge has to be relatively thick so that it can rub into the honeycomb and not have excessive wear at such knife edge. During such rub, the rotor knife edge is greatly heated with respect to the rest of the rotor. This causes a strong temperature gradient and high stress in such rotor which has caused rotors to crack and break off in certain engines, e.g. per crack 13 in FIG. 3 hereof, which (resulting gap) of course, can signifcantly impair the seal efficiency and operation of such engines.
In the prior art certain attempts have been made to improve the clearance control between rotor and outside annular stator, by blowing air against the outside surface of the stator, to reduce the annular air seal operating clearance. See, U.S. Pat. No. 4,460,311 to Trappmann et al. (1984), U.S. Pat. No. 4,525,998 to SWARZ (1985) and U.S. Pat. No. 4,662,821 to Kervistin et al. (1987). In another reference, blade tips abrade an outer annular slotted metal strip assembly, which is porous to admit air therethrough; see U.S. Pat. No. 3,970,319 to Carroll (1976). None of the above references attempts an improved lab seal between rotor knife blade and stator by providing a abradeable stator wear seal, that is protectively yielding to the rotor knife edges while providing a relatively non-porous stator seal surface and without relying upon clearance control mechanisms.
Accordingly there is a need and market for an improved lab stator seal that reduces rotor knife edge thermal gradients and stress and otherwise obviates the above prior art shortcomings.
There has now been discovered an improved lab stator rubbing surface or seal that reduces the wear, heat-up and resulting cracking of the rotor knife edge and reduces damage to the underlying honeycomb structure.